CORE M. Boucadair
Internet-Draft Orange
Intended status: Standards Track T. Reddy
Expires: August 30, 2019 McAfee
J. Shallow
NCC Group
February 26, 2019
Constrained Application Protocol (CoAP) Hop Limit Option
draft-ietf-core-hop-limit-03
Abstract
The presence of Constrained Application Protocol (CoAP) proxies may
lead to infinite forwarding loops, which is undesirable. To prevent
and detect such loops, this document specifies the Hop-Limit CoAP
option.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 30, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Hop-Limit Option . . . . . . . . . . . . . . . . . . . . . . 3
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4.1. CoAP Response Code . . . . . . . . . . . . . . . . . . . 4
4.2. CoAP Option Number . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
More and more applications are using Constrained Application Protocol
(CoAP) [RFC7252] as a communication protocol between involved
application agents. For example, [I-D.ietf-dots-signal-channel]
specifies how CoAP is used as a distributed denial-of-service (DDoS)
attack signaling protocol seeking for help from DDoS mitigation
providers. In such contexts, a CoAP client can communicate directly
with a server or indirectly via proxies.
When multiple proxies are involved, infinite forwarding loops may be
experienced. To prevent such loops, this document defines a new CoAP
option, called Hop-Limit (Section 3), which is inserted in particular
by on-path proxies. Also, the document defines a new CoAP Response
Code (Section 4.1) to report loops together with relevant diagnostic
information to ease troubleshooting.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers should be familiar with the terms and concepts defined in
[RFC7252].
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3. Hop-Limit Option
The Hop-Limit option (see Section 4.2) is an elective option used to
detect and prevent infinite loops when proxies are involved. The
option is not repeatable. Therefore, any message carrying multiple
Hop-Limit options MUST be rejected using 4.00 (Bad Request) error
message.
The value of the Hop-Limit option is encoded as an 8-bit unsigned
integer (see Section 3.2 of [RFC7252]). This value MUST be between 1
and 255 inclusive. CoAP messages received with a Hop-Limit option
set to '0' or greater than '255' MUST be rejected by a CoAP server/
proxy using 4.00 (Bad Request).
The Hop-Limit option is safe to forward. That is, a CoAP proxy which
does not understand the Hop-Limit option should forward it on. The
option is also part of the cache key. As such, a CoAP proxy which
does not understand the Hop-Limit option must follow the
recommendations in Section 5.7.1 of [RFC7252] for caching. Note that
loops which involve only such proxies won't be detected.
Nevertheless, the presence of such proxies won't prevent infinite
loop detection if at least one CoAP proxy which support the Hop-Limit
option is involved in the loop.
A CoAP proxy which understands the Hop-Limit option MAY be
instructed, using a configuration parameter, to insert a Hop-Limit
option when relaying a request which do not include the Hop-Limit
option.
The initial Hop-Limit value SHOULD be configurable. If no initial
value is explicitly provided, the default initial Hop-Limit value of
16 MUST be used. This value is chosen to be sufficiently large to
guarantee that a CoAP request would not be dropped in networks when
there were no loops, but not so large as to consume CoAP proxy
resources when a loop does occur. Lower values should be used with
caution and only in networks where topologies are known by the CoAP
client (or proxy) inserting the Hop-Limit option.
Because forwarding errors may occur if inadequate Hop-Limit values
are used, proxies at the boundaries of an administrative domain MAY
be instructed to remove or rewrite the value of Hop-Limit carried in
received messages (i.e., ignore the value of Hop-Limit received in a
message). This modification should be done with caution in case
proxy-forwarded traffic repeatedly crosses the administrative domain
boundary in a loop and so Hop-Limit detection gets broken.
Otherwise, a CoAP proxy which understands the Hop-Limit option MUST
decrement the value of the option by 1 prior to forwarding it. A
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CoAP proxy which understands the Hop-Limit option MUST NOT use a
stored TBA1 (Hop Limit Reached) error response unless the value of
the Hop-Limit option in the presented request is less than or equal
to the value of the Hop-Limit option in the request used to obtain
the stored response. Otherwise, the CoAP proxy follows the behavior
in Section 5.6 of [RFC7252].
Note: If a request with a given value of Hop-Limit failed to reach
a server because the hop limit is exhausted, then the same failure
will be observed if a less value of the Hop-Limit option is used
instead.
CoAP messages MUST NOT be forwarded if the Hop-Limit option is set to
'0' after decrement. Messages that cannot be forwarded because of
exhausted Hop-Limit SHOULD be logged with a TBA1 (Hop Limit Reached)
error response sent back to the CoAP peer. It is RECOMMENDED that
CoAP implementations support means to alert administrators about loop
errors so that appropriate actions are undertaken.
To ease debugging and troubleshooting, the CoAP proxy which detects a
loop SHOULD include its information (e.g., proxy name, proxy alias,
IP address) in the diagnostic payload under the conditions detailed
in Section 5.5.2 of [RFC7252]. That information MUST NOT include any
space character.
Each intermediate proxy involved in relaying a TBA1 (Hop Limit
Reached) error message SHOULD prepend its own information in the
diagnostic payload with a space character used as separator. Only
one information per proxy SHOULD appear in the diagnostic payload.
Doing so allows to limit the size of the TBA1 (Hop Limit Reached)
error message, and to ease correlation with hops count.
4. IANA Considerations
4.1. CoAP Response Code
IANA is requested to add the following entry to the "CoAP Response
Codes" sub-registry available at https://www.iana.org/assignments/
core-parameters/core-parameters.xhtml#response-codes:
+------+------------------+-----------+
| Code | Description | Reference |
+------+------------------+-----------+
| TBA1 | Hop Limit Reached| [RFCXXXX] |
+------+------------------+-----------+
Table 1: CoAP Response Codes
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This document suggests 5.06 as a code to be assigned for the new
response code.
Editorial Note: Please update TBA1 statements within the document
with the assigned code.
4.2. CoAP Option Number
IANA is requested to add the following entry to the "CoAP Option
Numbers" sub-registry available at https://www.iana.org/assignments/
core-parameters/core-parameters.xhtml#option-numbers:
+--------+---+---+---+---+------------------+-----------+
| Number | C | U | N | R | Name | Reference |
+--------+---+---+---+---+------------------+-----------+
| TBA2 | | | | | Hop-Limit | [RFCXXXX] |
+--------+---+---+---+---+------------------+-----------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Table 2: CoAP Option Number
5. Security Considerations
Security considerations related to CoAP proxying are discussed in
Section 11.2 of [RFC7252].
The diagnostic payload of a TBA1 (Hop Limit Reached) error message
may leak sensitive information revealing the topology of an
administrative domain. To prevent that, a CoAP proxy which is
located at the boundary of an administrative domain MAY be instructed
to strip the diagnostic payload or part of it before forwarding on
the TBA1 response.
6. Acknowledgements
This specification was part of [I-D.ietf-dots-signal-channel]. Many
thanks to those who reviewed DOTS specifications.
Thanks to Klaus Hartke, Carsten Bormann, Peter van der Stok, and Jim
Schaad for the reviews.
7. References
7.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[I-D.ietf-dots-signal-channel]
K, R., Boucadair, M., Patil, P., Mortensen, A., and N.
Teague, "Distributed Denial-of-Service Open Threat
Signaling (DOTS) Signal Channel Specification", draft-
ietf-dots-signal-channel-28 (work in progress), January
2019.
Authors' Addresses
Mohamed Boucadair
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
Tirumaleswar Reddy
McAfee, Inc.
Embassy Golf Link Business Park
Bangalore, Karnataka 560071
India
Email: kondtir@gmail.com
Jon Shallow
NCC Group
United Kingdom
Email: jon.shallow@nccgroup.com
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